Hello! I'm currently writing my PhD thesis about Portus Cale (modern Porto, Portugal) between the Fall of the Western Roman Empire and about the year 1000. I'm mainly focusing on trade from an archaeological standpoint, but I know written sources from this period pretty well, so maybe I can help you. The situation in Hispania after the death of emperor Theodosius (395) was already pretty catastrophic, even before the arrival of the Visigoths, so the change might not have been so radical as one might think. In the year 406, Suebi, Vandals and Alani cross the Rhine into Gaul, causing general anxiety. That might be one of the causes for an uprising in Britain around 406. In that year, a Roman general calling himself Constantine III claims the title of emperor and marches on Gaul, where he establishes his military base against Honorius, Theodosius's son and heir. The Hispanic provinces, however, remain loyal to Honorius, possibly because Theodosius had been born there, and some of his relatives and clients still remained there. In fact, some of his cousins seem to even have raised an ad-hoc militia to oppose Constantine's regular troops. The usurper thus sends a general by the name of Gerontius to the Peninsula, to suppress the loyalists. It is this same Gerontius that on 409 seems to have invited the Suebi, Vandals, and Alani to cross into the Peninsula (we still don't know if they had actually brokered a foederati pact). The best source to understand the Roman point of view of the settlement of Suebi, Vandals, Alani, and, later, Visigoths is Hydatius, bishop of Aquae Flauiae (modern Chaves, in northern Portugal). He seems to have been of a Roman influential family, and possessed a great deal of literary culture. He wrote a Chronicle in Latin, in which he narrates (and laments) the irreversible arrival of the so called barbarians to Hispania, culminating in the war of 459 between the Suebi and the Visigoths, in which the king Recchiarius died, and Hydatius (wrongly) assumed that the Suebic Kingdom was destroyed. Two things seem to be clear in his Chronicle, however. First of all, Hydatius doesn't have a very clear notion of what's happening outside his city. He never once makes a mention to the city of Bracara Augusta, which was the capital of his province of Gallaecia, and, afterwards, of the Suebic Kingdom. So the communication network might have broken down in the first decades of barbarian settlement. Secondly, even thought they are the uncontested political authority of Western Hispania, these Germanic peoples don't seem to, at first, consider the Hispano-Romans as their subjects. Rather, they relentlessly harass them during most of Hydatius' Chronicle. That is why 'mid-level bureaucrats' have to assume leadership roles they hadn't before. Hydatius himself, besides being a bishop, seems to have become the leader of his town, bartering with both Germans and Romans. Finally, the civic administrative structures of the Roman Empire were not intentionally destroyed by the Visigoths. Rather, they were privatised by the military elite - the king and his retinue or, when obsolete, ignored. About this last point, I would highly recommend Chris Wikham's "Framing the Early Middle Ages" (2005), in which he dedicates a chapter to the emergence of the Visigothic state.